In gases discharged from internal combustion engines such as diesel engines, various types of combustion apparatuses and the like, a large amount of particulate matter (PM) mainly composed of soot is included. When this PM is discharged as it is to the atmosphere, environmental pollutions are caused. Therefore, a diesel particulate filter (DPF) for collecting the PM is mounted on an exhaust system of an exhaust gas.
As such a DPF, there has been used, for example, a honeycomb structure “including porous partition walls to partition and form a plurality of cells which become through channels of a fluid (an exhaust gas and a purified gas) and an outer peripheral wall positioned in an outermost periphery, and further including plugged portions in open frontal areas of predetermined cells in an end surface on an inflow side of the fluid (the exhaust gas) and open frontal areas of the remaining cells in an end surface on an outflow side of the fluid (the purified gas)”.
There has been a problem that when the PM in the exhaust gas is collected by using such a honeycomb structure, the PM penetrates into the porous partition walls to clog pores of the partition walls, and a pressure loss rapidly increases sometimes.
A filter has been suggested in which to suppress the increase of a pressure loss, a collecting layer for collecting a PM is disposed on the surfaces of partition walls, and the collecting layer prevents the penetration of the PM into the partition walls, to suppress the rise of the pressure loss (e.g., see Non-Patent Document 1).
Heretofore, as a regulation on an automobile exhaust gas, the regulation by means of a mass of the PM has been performed. In recent years, however, the introduction of the regulation on the number of the PMs has been investigated. In this case, it is necessary to securely collect the PM having small particle diameters. It is known that the PM having the small particle diameters is collected by the surfaces of pores which are present in a filter, mainly by diffusion (e.g., see Non-Patent Document 2).
Heretofore, when the honeycomb structure including the collecting layer is prepared, the collecting layer has been formed by immersing the honeycomb structure into a collecting layer forming slurry (a collecting layer forming raw material) or pouring the collecting layer forming slurry into cells of the honeycomb structure, to coat porous partition walls (a partition wall parent material) with the collecting layer forming slurry, followed by firing. Moreover, when a porous film having smaller pore diameters and smaller thickness than the porous partition walls is formed on the surfaces of the partition walls, it has been necessary to set the particle diameters of ceramic particles constituting the porous film to be smaller than the pore diameters of the partition walls. However, in this method, there has been a problem that the collecting layer forming slurry penetrates into the pores of the partition walls (the partition wall parent material) of the honeycomb structure, and an initial pressure loss in the case of the circulation of the exhaust gas through the obtained honeycomb structure becomes high.
Furthermore, in the case of a honeycomb structure made of cordierite, aluminum titanate or the like, there has been a problem that a slurry penetrates into micro cracks formed in the honeycomb structure or the like, and a thermal expansion coefficient or the like of the obtained honeycomb structure or the like becomes high.
On the other hand, a method has been suggested in which pores of a porous support material are charged with “a substance which can be removed later”, the pores are clogged, and then the surface of the porous support material is coated with a slurry including ceramic particles having small particle diameters (e.g., see Patent Documents 1 to 3). Examples of “the substance which can be removed later” can include combustible substances (Patent Document 1). When the combustible substances are used, the combustible substances can be burnt and removed later in a firing step. Moreover, examples of “the substance which can be removed later” can include water and alcohol (Patent Documents 2 and 3). When the water or alcohol is used, the surface is coated with the slurry and then dried, whereby the water or alcohol can be removed.
Moreover, a method has been suggested in which a ceramic porous film (a collecting layer) is formed on the surface of a porous support material by use of fine particles mainly composed of an oxide such as alumina or zirconia (e.g., see Patent Document 4). Specifically in the method, the ceramic porous film is formed on the surface of the porous support material made of a porous ceramic by use of a porous film forming coating material containing the fine particles mainly composed of the oxide having controlled average primary particle diameter, tap bulk density and average secondary particle diameter in the coating material (the average secondary particle diameter when the material is dispersed in a dispersion medium), and containing the dispersion medium mainly composed of water. A viscosity of the material is controlled to be 2 mPa·s or larger and 1000 mPa·s or smaller.
Furthermore, a method has been suggested in which a collecting layer forming slurry prepared by further adding a pore former and water to the same material as that of a honeycomb formed body is sprayed on the honeycomb formed body, to deposit the collecting layer forming slurry on partition walls of the honeycomb formed body, followed by drying and firing, thereby disposing the collecting layer on the honeycomb formed body (e.g., see Patent Document 5).
In addition, a method has been suggested in which a slurry made of a bonding material mainly composed of an inorganic fibrous material longer than pore diameters of porous partition walls and silica or alumina is deposited on the surfaces of the partition walls, followed by drying and firing, thereby forming a porous film (a collecting layer) on surface layers of the partition walls (e.g., see Patent Document 5).